In phase shift keyed systems, several factors contribute to the degradation of the signal and lead to an increase in the system error rate at a particular carrier to noise ratio, some of these occur as the result of the propagation of the microwaves from a transmitter to a receiver, and some as a result of the apparatus used in the receiver. An amplitude versus frequency slope across the symbol-rate band width of a transmission system, which is usually caused by frequency selective fading across the pass band of the channel, causes distortion of the received signal which results in a reduced pulse amplitude out of a demodulator apparatus used in the receiver due to the generation of quadrature components during demodulation. Another factor is the linear group-delay variation resulting from reflections taking place during the transmission of the signal from the transmitter to the receiver. The receiver then receives both a direct signal and an indirect, reflected signal and there is a phase difference between these two signals which, on recombination may reduce the absolute signal level of the received signal but which, in any event, generates quadrature components of the received carrier vector in the demodulator apparatus which again reduces the pulse amplitude out of the demodulator so that, it is more difficult to differentiate between noise and the received information. Both of the above factors are caused by conditions subsisting during the transmission of the signal from the transmitter to the receiver.
In the past, the receiver has included means to recover a carrier signal from the incoming phase shift modulated signal and the demodulator apparatus has applied a fixed phase offset to the recovered carrier signal before applying it to the demodulator apparatus as a phase reference for all phase locked states of the recovered carrier. In four-phase phase shift keyed systems, there are four locked states of the recovered carrier signal. In the past, this offset between the recovered phase of the carrier and the phase to be used for demodulation purposes has been determined experimentally in the laboratory and then pre-set into the equipment. In practice, the optimum phase offset differs from the laboratory estimation and, this leads to a phase error between the incoming signal and the reference signal used for demodulation purposes. This phase error reduces the level of the demodulated signal and this again reduces the demodulated signal to noise ratio and this increases the system error rate. Finally, the phase offset applied by the demodulator apparatus varies as a result of the carrier signal recovery means of the demodulator apparatus, tracking frequency variations of the received signal. Frequency variations occur in the received signal and frequency variations also occur in a voltage controlled oscillator forming part of the carrier recovery means due to variations in the temperature. These variations between the frequency of the received signal and the recovered carrier signal result in changes in the phase offset which are proportional to the frequency error.
It can be shown theoretically that about 40% of the signal degradation measured in dB, caused by amplitude slope and about 60% of the signal again measured in dB, caused by the linear group-delay variation can be recovered by varying the phase of the recovered carrier at the receiver for a given error rate. Further, degradation caused by phase offset changes originating in the demodulator apparatus could be recovered by varying the phase offset applied to the recovered carrier signal by the demodulator apparatus.
It is therefore a primary object of this invention to provide a demodulator apparatus in which the phase offset applied to the recovered carrier signal is variable.
It is another object of this invention to provide a demodulator apparatus in which the phase offset of the recovered carrier signal is varied to optimise the amplitude of the output of the demodulator.
It is a further object of the invention to provide a demodulator apparatus in which the demodulated output signal is monitored and the phase offset varied to maximise the demodulated output signal.